Current transformers (CTs) are widely used in the electric power industry to measure line current for protection and metering. The line current is applied to a primary coil of the CT, and a reduced-magnitude version of the line current is produced on a secondary coil of the CT. This reduced-magnitude version of the line current is suitable for electronic measurement, monitoring, and control and protection applications of power equipment in the electrical network. Typically, during faults and disturbances, due to high electrical current flowing through the CT, the core of the CT saturates. This results in generation of a distorted secondary signal as an output at the secondary terminals of the CT, thereby having a condition of incorrect representation of the electric current flowing in the electrical network. This core saturation phenomenon can adversely affect all the measuring, monitoring and protection applications that rely on the current output from CT. Because the output of the CT may not indicate the true current in the electric network during the fault and disturbance condition, adequate protection and control decisions implemented through the protection devices may get affected, thereby resulting in damage of equipment connected in the electrical network.
Moreover, signal conditioning and measurement circuits typically need voltage and/or current as an input and hence the voltage and/or current of the secondary of the CT are signal conditioned to an appropriate voltage and used for measurement and monitoring purposes. Conventional approaches involve connecting a static burden resistance to the secondary of the CT. This burden resistance sets the operating range of the CT. The lower operating range decides the accuracy specification because the voltage across the burden resistance is small. The higher operating range is decided based on the signal conditioning circuit and saturation limit of the core. With increasing primary current, flux in the magnetic core increases, pushing it towards saturation. Furthermore, with an increase in flux, the non-linear behaviour posed by the magnetic material of the core increases, thereby resulting in higher errors at higher operating ranges.
FIG. 1 depicts a part of an equivalent circuit 100 of a conventional current transformer having a burden resistor rb connected with a secondary coil represented by a resistor rCoil, of the current transformer. The excitation quantity for the system is the primary current IP. The governing equations for the system are:
      Φ    =                                        N            p                    ⁢                      I            p                          -                              N            s                    ⁢                      I            s                              R            Φ    =                            I          S                *                  (                                    r              b                        +                          r              Coil                                )                            (                              N            S                    *          ω                )            Where: Φ—core flux, IP—primary current, NP—number of turns of the primary, which are typically=1 for a current transformer application, IS—secondary current, NS—number of turns of the secondary coil, R—reluctance of the magnetic core, rb—secondary burden resistance, rCoil—resistance of the secondary coil and ω—frequency of excitation. Solving these two equations we can derive the equation for Φ as a function of the excitation, that is, a function of the primary current, using the equation below:
  Φ  =            [                        N          p                ⁢                  I          P                *                  (                                    r              b                        +                          r              Coil                                )                    ]              [                        (                                    Ns              2                        *            ω                    )                +                  R          *                      (                                          r                b                            +                              r                Coil                                      )                              ]      In a conventional scenario, the burden resistance rb is kept constant. With all other quantities as constant, it is seen that Φ is proportional to IP and thus increases with an increase in IP. Thus the flux Φ increases thereby, climbing upward in the B-H curve associated with the current transformer. Above a certain limit, the flux Φ saturates causing distortions in the secondary voltage and measurements. Due to this there still exists a non-linearity in the conventional burden resistor based systems. Moreover, in case of higher operating primary currents, the current through the burden resistor rb increases. This increases the power dissipation and the temperature of the burden resistor rb. With further increase in temperature, resistance of the burden resistor rb changes thereby, affecting accuracy of measurements.
Conventional approaches are typically aimed at detecting saturation in a current transformer, wherein if saturation is detected, the information is used to restrain/adapt a protection device's measurements dependent on the data measured by the current transformer by employing curve fitting, look up tables, etc. Some other methods typically use artificial intelligence techniques like neural networks to detect this saturation wherein huge amount of past data is required to train the neural network. However, these conventional approaches are largely aimed at correction of the errors occurring as a result of core saturation. Moreover, calibrating such errors is tedious as the measurements are dependent on material properties of the core which vary with temperature. Furthermore, calibration of these errors is difficult because it requires determination of fine points which will also vary with materials and environmental factors.
Therefore, it is an object of the present disclosure to provide a device of the aforementioned kind that extends an operating range of a current transformer by preventing the transformer core from saturating thereby, resulting in an increased accuracy of measurement at higher primary currents and protection of downstream electronics at higher operating currents.